1. Field of the Invention
The present invention is directed to acoustic noise confinement and reduction in general, and in particular to the confinement of sound to the region of use of loudspeaking or handsfree telephones, computer terminals, etc., as are commonly used in open-plan offices, cubicles, booths and so forth, particularly where the partitioning walls do not extend fully to the ceiling. By reducing the loudspeaker radiated sound beyond the edge of partitions, the noise in neighboring booths may be reduced to the leakage of the sound locally produced, such as the voice of an occupant on the telephone. Thus, noise from loudspeaking telephony becomes comparable to that from ordinary handset use.
2. Description of the Related Art
Heretofore, various noise suppressing systems have been proposed. Several examples of such previously proposed noise suppressing systems are disclosed in the following U.S. Pat. Nos.:
______________________________________ U.S. Pat. No. Patentee ______________________________________ 4,463,222 Poradowski 4,934,483 Kallergis 5,289,147 Koike et al. 5,381,473 Andrea et al. 5,388,160 Hashimoto et al. 5,432,857 Geddes 5,408,532 Yokota et al. ______________________________________
The Poradowski U.S. Pat. No. 4,463,222 discloses a noise cancelling transmitter for voice communication comprising a casing having a principle surface opposed to the mouth of the user and three side surfaces facing upwardly, laterally and downwardly when the principle surface is so opposed. Noise cancelling openings in the three side surfaces communicate noise to the back of a diaphragm in the transmitter microphone. Openings in the principle surface communicate both noise and the speaker's voice to the front of the diaphragm. The noise acts on both sides of the diaphragm and is thus cancelled, while the voice acts only on one side of the diaphragm and vibrates it.
The Kallergis U.S. Pat. No. 4,934,483 discloses a method of reducing the overflying noise of airplanes having a propeller driven by a piston engine. The propeller is arranged on the engine shaft in such a way that positive components of the engine sound pressure fall on negative components of the propeller sound pressure. It is preferable to use an engine/propeller combination in which the number of engine ignitions per revolution of the propeller shaft divided by the number of the propeller blades is an integer, preferably being equal to 1.
The Koike et al. U.S. Pat. No. 5,289,147 discloses an image forming apparatus which includes a housing, a mechanism, mounted in the housing, for forming images on a medium, and an operation panel formed on the housing, the mechanism being driven in accordance with operating instructions input from the operation panel by an operator. The apparatus further includes a microphone, provided in the housing, for detecting a noise generated by a driving of the mechanism, and a noise cancelling unit for outputting an acoustic wave to an area adjacent to the operation panel of the housing, the acoustic wave being generated based on the noise detected by the microphone so that the acoustic wave and a noise present in the area cancel each other out, whereby the noise present in the area is reduced.
The Andrea et al. U.S. Pat. No. 5,381,473 discloses an apparatus for reducing acoustic background noise for use with a telephone handset or a boom microphone device or the like. The apparatus includes first and second microphones which are arranged such that the first microphone receives a desired speech input and the background noise present in the vicinity of the speech, and the second microphone receives substantially only the background noise. The background noise from the second microphone is converted into a corresponding electrical signal and subtracted from a signal corresponding to the speech and background noise obtained from the first microphone so as to produce a signal representing substantially the speech.
The Hashimoto et al. U.S. Pat. No. 5,388,160 discloses a noise suppressor in which a noise signal detected by a first detector is inputted to an adaptive filter and a FIR filter. An output signal of the adaptive filter is reproduced by a speaker. The signal reproduced by the speaker and a noise signal from a noise source are detected by a second detector. The signal detected by the second detector is band-limited by a filter circuit and sent to a LMS computing circuit. The LMS computing circuit updates a coefficient of the adaptive filter so as to minimize an output signal of the filter circuit in response to an output signal of the FIR filter and an output signal of the filter circuit.
The Geddes U.S. Pat. No. 5,432,857 discloses an active muffler for use in motor vehicles comprising a sensor, an electronic control responsive to the signal generated by the sensor for producing a drive signal delivered to a transducer which emits cancellation pulses phased 180.degree. from the sound pressure pulses passing through a conduit, where both front and rear sides of the transducer are acoustically coupled to the conduit to improve the efficiency of the transducer operation. Preferably, the acoustic coupling comprises an enclosed chamber including a port for communicating with the conduit which can be tuned to resonate at predetermined frequencies. When both sides of the transducer are so coupled to the conduit, the transducer has increased efficiency over a broad band of frequencies, and the frequency band can be broadened at the low end as required to accommodate the frequencies generated by a source of noise. A tandem transducer mounting arrangement constructed according to the teachings of this invention reduces vibration of the housing. The transducer mounting arrangement is particularly suitable for use in adapting noise cancellation techniques to replace passive mufflers on motor vehicles.
The Yokota et al. U.S. Pat. No. 5,408,532 discloses the use of an ignition pulse signal which is transformed into a single vibration noise source signal (primary source) so as to obtain a frequency spectrum composed of 0.5.times.n order components which is converted into a cancelling signal after being subjected to the sum of convolution products processed with filter coefficients of an adaptive filter. Further the cancelling signal is converted into a cancelling sound by a speaker and outputted to the passenger compartment to cancel vibration noise at a noise receiving point. The state of noise reduction is detected as an error signal by a microphone and the error signal is inputted to an exponential averaging circuit where the error signal is exponentially averaged with previous error signals by a trigger signal of the primary source from a trigger signal generating circuit. The error signal, as a result of this averaging, is compressed and then outputted to a least mean square (LMS) operational circuit. In the LMS operational circuit, the filter coefficients are updated based on the primary source inputted via speaker/microphone transmission characteristic correction circuit and the compressed error signal.